Transvenous cardioversion and defibrillation leads typically employ cardioversion or defibrillation electrodes taking the form of elongated metal coils. These coils may be applied to the exterior surface of the lead body, as disclosed in U.S. Pat. No. 4,934,049 issued to Kiekhafer et al.
One problem associated with defibrillation coils of the type discussed in the '049 patent involves the occurrence of tissue in-growth around the coil structure of the electrode. This is particularly true when the coil electrode is formed around the lead body so as to create an enlarged profile at the location of the coil as compared to the rest of the lead body. Tissue attachment that occurs proximal to the electrode structure makes it difficult to extract distal end of the lead, including the enlarged coil.
One way to reduce the problem of tissue in-growth involves providing defibrillation coils that are isodiametric with respect to the lead body. Such isodiametric electrode coils may be molded into the electrode body or the coils may be machined to provide a flush surface. This is described in U.S. Pat. No. 4,161,952, issued to Kinney et al. Similarly, U.S. Pat. No. 5,957,970 to Shoberg discloses an isodiametric defibrillation lead manufactured by removing a portion of an extruded tubular lead body in the region of the coil so that the electrode is flush with the surface of the lead. The 6944 Model lead commercially available from the Medtronic Corporation provides an isodiametric lead of this design.
Although isodiametric coil electrodes reduce problems associated with extracting an enlarged lead portion from ingrown tissue, other problems still exist related to ingrowth. For example, tissue commonly attaches around the coils of a defibrillation electrode, further increasing the difficulty associated with lead extraction.
Various methods have been attempted to overcome the problems associated with tissue in-growth around coil electrodes. One solution disclosed in the '049 patent referenced above involves injecting silicone rubber into the spaces between the individual coils of an electrode. The resulting thin coating of silicone rubber surrounding the exterior of the coils of electrode minimizes tissue in-growth between the filars of the coils, while leaving a portion of the coils exposed to deliver electrical stimulation to a patient.
Another approach to preventing tissue in-growth is disclosed in U.S. Pat. No. 5,090,422, which describes the use of a biocompatible porous materials such as woven, porous polyurethane and porous polytetrafluoroethylene that may be used to cover an electrode surface. The material is insulative when dry, but becomes conductive when bodily fluids penetrate the pores of the material. The porous covering is of adequately small pore size and fibril length to preclude substantial tissue in-growth.
Yet another method of preventing tissue in-growth is disclosed in U.S. Pat. No. 5,609,622, which describes coating a lead with a porous Polytetrafluoroethylene (PTFE) layer such as may be formed of expanded PTFE (e-PTFE), and which has a pore size of less than 10 microns or smaller so that the pore size is very small, and tissue in-growth is prevented.
While the foregoing approaches are directed to preventing tissue in-growth, it may be noted that some controlled tissue attachment to a lead body or electrode may actually be beneficial in stabilizing the lead, and allowing the electrode to maintain a position at a desired implant site. For example, in leads carrying defibrillation electrodes, a change in the coil placement may increase the defibrillation thresholds. Similarly, in pace/sense applications, a shift in electrode position may alter the pacing threshold and affect capture. Promoting tissue growth for stabilizing prosthesis and other implanted structures has been disclosed in the prior art. For example, U.S. Pat. No. 5,035,713 discusses use of a re-entrant biocompatible material such as polyethylene, polyethylene teraphthalate, polypropylene, polysulfone, polylactic acid and polydioxanone to promote selective tissue in-growth to stabilize an implanted structure. Similarly, U.S. Pat. No. 5,833,664 discusses promoting tissue attachment to intrabody prosthetic devices, such as catheters, to effect improved stability of the prosthesis/tissue opening site interface.
Yet a further concern associated with the use of medical electrical leads, and in particular, the use of leads carrying coiled electrode structures, involves the possibility of a coil causing abrasion to an adjacent lead structures. For example, a coiled electrode adapted for use in the right atrium may lie in close proximity to a second lead carrying an electrode placed in the right ventricle. The coiled electrode may contact the second lead, creating abrasions in the external lead surface so that an internal conductor comes in contact with body fluids. This condition may ultimately result in lead failure. Also, in applications in which leads are positioned within relatively smaller vasculature structures, such as in coronary veins, lead-to-lead contact, or lead-to-tissue contact can be exacerbated, thus increasing the likelihood of abrasion.
Therefore, what is needed is an improved lead structure that minimizes the potential for the abrasion of adjacent structures. The lead ideally promotes selective, controlled tissue attachment to stabilize lead placement, while preventing tissue ingrowth that would prevent lead extraction.